Encryption not possible on Oneplus 6T

  • My phone is Oneplus 6T and Android version is 10.

  • K-9 Mail version is 5.732 downloaded form Github release.

  • Openkeychain version is 5.6.2 installed form F-Droid.

My issue is that I can connect to Openkeychain and use the key to decrypt email, but sending encrypted email is not possible. I wonder how to fix this issue.